It is a common oilfield practice to inject gas into an oil reservoir for pressure maintenance. Relatively inexpensive gas may be added to a reservoir at sufficient rates to increase reservoir pressure resulting in an increased oil production rate while reducing or eliminating aquifer influx and associated water production. Accomplishing a significant pressure increase requires that a large, cost effective, source of inert gas be available. Surface generation of low pressure flue gas has been employed by industry as a source of inert gas, however, it is expensive to treat the flue gas in order to remove acids in the gas prior to compressing the gas for injection into the reservoir. These costs can be avoided by generating the gas under pressure.
Under some circumstances, it is desirable to heat the injected gas. For example, heat has been used to increase the mobility of the oil by decreasing the viscosity of the oil in the formation, increasing the volume of the oil, or increasing the rate of imbibition of flooding fluids. Steam or an oxidizing gas is often injected into formations bearing highly viscous oil. The oxidizing gas can be used to burn some of the oil in situ, thereby providing an additional heat source. However, in other applications, such as when the oil is not highly viscous, heat and pressure may be applied to the formation by injecting heated gas, such as nitrogen, carbon dioxide, or recycled produced gas.
If the gas for injection is heated at the surface, considerable heat is lost as the gas flows through the flowline, well tubing and casing, and into the formation. Thus, it is more efficient to heat the gas inside the well, adjacent to the formation into which the gas is to be injected.
If the reservoir to be heated has been under production for a considerable length of time, it may also be more economic to convert existing producer wells to injector wells. Significant costs can also be avoided by using existing surface and downhole hardware in the converted wells. However, prior art downhole burners require the use of specially fabricated wellhead hardware, downhole hardware, and a specially completed well, and cannot be easily used in already-completed wells.
Prior art burners have not been designed for prolonged operation or to provide the combined benefit of inert gas generation and superheating of the injected oxygen containing gas stream. Injected gas is often cool, resulting in low reactivity and less efficient reaction of the oxygen. Superheated gas injection assures maintenance of a hot reactive system from the burner out to the unaffected liquid hydrocarbon containing formation.
Prior art burners have traditionally used electrical igniters to start combustion within the well, which requires that electrical wires be run alongside the fuel gas and oxidizer tubing down to the burner, thus requiring that the wires penetrate any packer device used to prevent reservoir fluids from flowing back up the well. These electrical wires are a source of leakage of the packer device.
Another disadvantage of prior art burners is that they lack the plug profile necessary to use wireline retrievable plugs. The use of wireline retrievable plugs provides the opportunity for the burner to be inserted (commonly called snubbing) into the well without "killing" the well, thus saving cost, and damage to the formation containing the well, as well as providing a safer environment for the well operators as the burner is being inserted into and removed from the well. This also facilitates repair of the burner nozzle separate from repair of the shroud.
Still another limitation of prior art burners is that the entire burner, including the concentric or parallel fuel and air conduits must be inserted at the same time, since the burner must be assembled at the surface.
Thus, there is a need for a burner to generate inert gas for injection into oil reservoirs. There is also a need to heat the injected gas utilizing existing and conventional well casing, wellhead, and downhole hardware with such a burner. A further need is for a burner that does not require electrical ignition, and for a burner that can be inserted into a well without killing the well. The present invention meets these and other needs in the art.